1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly, to a method of fabricating a semiconductor device in which germanium ions are implanted before implanting P-type ions in a channel-stop ion implantation process, such that the lattice structure of the surface of a shallow-trench isolation region is maintained to thereby allow a deeper penetration of the implanted P-type ions (boron ions), and to prevent the P-type ions from being outwardly diffused according to an increased lattice scattering phenomenon generated upon thermal processing.
2. Discussion of the Related Art
An imaging device for converting an optical image into an electrical signal may be realized by a charge-coupled device or a CMOS image sensor. A CMOS image sensor uses CMOS fabricating technology.
CMOS image sensors include a photosensitive portion for sensing light and a logic circuit for processing the sensed light into an electrical signal. The unit pixel of the CMOS image sensor may be composed of one photodiode and four NMOS transistors. Typically, the four transistors are a transfer transistor, for transferring an optical charges collected in the photodiode to a floating node, a reset transistor, for setting the potential of the node to a desired value, discharging charges, and resetting the floating node, a drive transistor, functioning as a source follower buffer amplifier, and a select transistor, functioning as a switch for addressing. Among these transistors, the transfer transistor and the reset transistor are formed of native NMOS transistors having depletion modes or low threshold voltages, to improve charge transfer efficiency and reduce a charge loss (voltage drop) in the output signal. In the unit pixel of the CMOS image sensor, the potential barriers of the transfer transistor and the reset transistor are controlled such that the reset of the floating node is performed through the transfer transistor and the reset transistor, and excessive charges in a saturation region flow into a power line through the transfer transistor and the reset transistor.
FIGS. 1A-1C illustrate a method of fabricating a CMOS image sensor according to the related art.
Referring to FIG. 1A, a trench is formed in a silicon substrate 101 to form a shallow-trench isolation region 102.
Referring to FIG. 1B, P-type ions, such as BF2 or boron, are implanted into the shallow-trench isolation region 102 of the silicon substrate 101 while covering the photodiodes 103 with a photoresist 104. The N-type photodiode and the shallow-trench isolation region 102 are separated from each other, and thus leakage in the interface of the shallow-trench isolation region 102 is removed and isolation is improved.
Referring to FIG. 1C, because the boron is light in weight it is outwardly diffused upon a post-thermal process. Thus, the boron cannot perform its function well because upon thermal processing the P-type region 105 becomes wider, and the photodiode 103 becomes narrower.